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London, England (CNN) -- The oceans have become so depleted by over-fishing, pollution and climate change that they can only be saved by a large global network of reserves, according to a growing consensus among marine scientists.

Campaigners say that sea life -- particularly at the top of the food chain -- is suffering to such an extent that there will eventually be no fish left if action drastic action is not taken to protect the oceans.

More than 70 percent of the world is covered by oceans. There are currently more than 4,000 marine protected areas covering just over 1 percent of the oceans, but the vast majority of reserves have only limited protection.

According to Professor Callum Roberts, of the University of York, one of the leading campaigners and author of The Unnatural History of the Sea, only about 0.1 percent of the sea is completely protected from all exploitation. This should be between 25 and 45 percent to give marine species the best chance of recovery, he said.

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We are asking too much of our oceans and not paying enough attention to their long-term health.--Jay Nelson, director of Global Ocean Legacy, Pew Environment Group

The Global Ocean Legacy, a project of the Pew Environment Group, issued a statement to mark World Oceans Day in June signed by 257 marine scientists in 37 countries calling for a large network of highly protected no-take reserves.

Prof Roberts told CNN: "There's strong and ample evidence that the oceans' eco systems are in trouble and need protection.

"Fishing now reaches every corner of the world's oceans, so the only refuges are those we have chosen to create.

"In the future climate change is going to loom ever more heavily as a factor in damaging marine life. The only way the oceans can remain resilient to climate change is by establishing more protection."

According to Greenpeace, 90 percent of the large predator fish stocks are gone or in trouble and 90 percent of exploited fish stocks in the European Union are in trouble.

David Ritter, head of biodiversity at Greenpeace UK, told CNN: "There's no doubt that the oceans are in crisis.

"All over the world, we are seeing a systematic degradation of the marine environment.

"In some fisheries as many as 80 percent of fish are simply thrown away as by-catch.

"Fish stocks are collapsing, taking out first the top predators, such as sharks, tuna and swordfish, and moving down the food chain.

"We now have a lot more small fish and crustaceans because their predators are gone, but eventually they will be gone too. We could end up with a situation where there's nothing left but worms."

The Pew Environment Group set up its five-year Global Ocean Legacy project in 2007 with the intention of persuading governments to set up four very large marine reserves: 410,000 sq miles in the Coral Sea off Australia; a 246,000 sq mile area around the Kermadec Islands near New Zealand; the Mariana Trench, the world's deepest trench in U.S. waters in the Pacific; and 210,000 sq miles around the Chagos Islands in the Indian Ocean.

So far, two of these targets, the Mariana Trench and the Chagos Islands, have been successful, and the other two campaigns are ongoing.

The Chagos Islands, protected in April this year by the UK government, became the world's largest marine reserve.

Jay Nelson, director of Global Ocean Legacy, said: "We are asking too much of our oceans and not paying enough attention to their long-term health.

"For 150 years, it's been accepted that iconic or important places on land should be protected, but in the sea it's a relatively new idea.

"Fisheries are getting further and further afield and we are getting more unusual species of fish from parts of the ocean we would not have dreamt of going to 30 years ago.

"The emergence of fish like Chilean Seabass is an example. The species is actually Patagonian Toothfish, but that's not a good name for marketing. These fish take 80 years to mature, and you can't sustainably eat a fish that's 80 years old. We are already having to go further and further south into the Antarctic Ocean to find it.

In 2009, the UK introduced a Marine and Coastal Access Act, creating new Marine Conservation Zones, although the act did not go as far as some campaigners had hoped because no target was set for the percentage of UK waters to be protected.

Other countries which have made significant progress on creating marine reserves include Germany, Australia, South Africa, the United States and the Federated States of Micronesia, according to Prof Roberts.

However, the majority of the world's oceans are international waters, where there is no single body responsible for their protection.

Nelson said: "Reserves are relatively easy to set up in national waters because governments can take unilateral action.

"However, it's more complicated in international waters because they require co-operation across nations. Surveillance and enforcement in international waters is particularly challenging."

Nelson said that one 36,000 sq mile reserve had been set up last year in international waters around the South Orkney Islands in the Southern Ocean, and that progress was being made towards others.

Any move towards creating marine reserves comes against resistance from some parts of the fishing industry, which says its livelihood is at stake.

Nelson said: "Marine reserves upset the status quo that all oceans should be open to everyone all the time, and to change that is a threat to some of those who use the oceans.

"However, our experience is that once marine reserves have been set up, everyone sees the benefits, even those who were opposed to them.

"Once reserves have been created we see the fish populations in neighboring fisheries increase and the size of individual fish increase.

"Our oceans have been so depleted that scientists don't even know what they should look like when they're healthy. However, where reserves have been set up, we are beginning to see that."